In recent years, the information society has been increasingly developed, and the demand for higher speed, higher capacity, smaller size, lighter weight, or the like of, for example, a personal computer, a cellular phone, or the like has been increased. Therefore, semiconductor devices such as a large-scale integrated circuit (also referred to as a large scale integration (LSI)) and a central processing unit (CPU) need higher integration, higher operation speed, and lower power consumption.
Power consumption of the semiconductor device is substantially equal to the total of a power consumption generated in an operation state and a power consumption generated in a stop state (hereinafter referred to as a standby power) of the semiconductor device.
The standby power can be classified into static standby power and dynamic standby power. The static standby power is power consumed by generation of leakage current between a source and a drain, between a gate and the source, and between the gate and the drain in a state where voltage is not applied between the electrodes of a transistor in the semiconductor device, that is, in a state where a voltage between the gate and the source is approximately 0 V. On the other hand, the dynamic standby power is power which is consumed when voltages of various signals such as a clock signal or a power supply voltage continues to be supplied to a circuit in standby state.
Although a microfabrication technique has been developed to obtain higher operation speed of a semiconductor device, when microfabrication is advanced, a channel of a transistor becomes smaller in length and an insulating layer typified by a gate insulating layer has a smaller thickness. Therefore, leakage current of the transistor is increased and the dynamic standby power tends to be increased. As a method for obtaining higher operation speed of a semiconductor device without microfabrication, a method is given in which a transistor is formed using a substrate in which an insulating region is provided over a first semiconductor region, and a second semiconductor region is provided over the insulating region (also referred to as an SOI substrate) (e.g., Patent Document 1).